A letter that represents a quantity is called a literal number or simply literal.
The numerals $0$, $1$, $2$, $3$ and etc. are arithmetical symbols and they are used as numbers to represent quantities in basic mathematics.
The same concept is inspired to introduce algebra but here the symbols are used to represent quantities instead of numbers. However, the symbols like $+$, $-$, $\times$, $÷$ and some other special symbols cannot be used to represent quantities. Hence, the letters like alphabets belongs to English, Greek and etc. are used primarily to represent quantities.
Therefore, the letters which represent numbers are called literal numbers or literals. Literals are basic algebraic symbols, used in algebra to represent any quantity but we have to specify that a letter is used to represent a particular quantity to avoid others’ confusion.
Observe the following examples for understanding the concept of literals in algebra.
The number of days of a week is $7$. Take any letter to represent this quantity.
For example, $d = 7$.
Now, just express the letter $d$ wherever you want to represent number of days of a week. The letter $d$ represents a quantity. So, the letter $d$ is called a literal in algebra.
The angle of a straight line is $180^\circ$.
$\alpha = 180^\circ$
The letter alpha is called a literal number.